The present invention relates to radio frequency (RF) reception apparatus and, more particularly, to a novel ultra-low-noise RF preamplifier having unconditional stability and large dynamic range.
It is well known that many forms of RF reception equipment, for communications, magnetic resonance imaging (MRI) and the like uses, can benefit from a small-signal RF preamplifier having relatively high gain (in excess of 20 dB.) along with a ultra low noise figure (less than 0.3 dB.). at an operating frequency in the approximately five octave range from about 15 MHz. to about 500 MHz. It has been possible, since the latter part of the 1970s, to provide bipolar transistor RF preamplifiers having a low noise figure (NF) between about 0.8-1.0 dB. over most of this frequency range. Very low noise preamplifiers, with noise figures in the range of 0.4-0.8 dB., were then available utilizing gallium-arsenide MESFETs, or GaAsFETs, for frequencies only above about 100 MHz. (See, for a general overview, "VHF Preamplifiers", GH Krauss Ham Radio, December, 1979.) While the use of microwave GaAsFETs in very low noise preamplifiers has now become relatively common place, above 50 MHz., a continuing problem is the potential instability (i.e., a stability factor K of less than 1) of all hitherto-described preamplifiers in which a common-source input stage configuration is utilized. Since the common-source configuration is the only configuration presently known to provide a truly low noise figure, the potential instability, particularly at the frequency of interest, signifies a RF preamplifier which tends towards oscillation at some unknown frequency, due to the reactive nature of at least one of the source impedance and load impedance seen by that RF preamplifier. This is especially so where relatively narrow-bandpass filters are also utilized before and/or after the RF preamplifier in many applications where the extreme sensitivity of the RF preamplifier and/or the subsequent reception apparatus can be adversely affected by transmission of signals at undesired frequencies. Accordingly, it is highly desirable to provide an unconditionally-stable, ultra-low-noise RF preamplifier (having a noise figure less than about 0.3 dB.) at a frequency of interest in the 15-500 MHz. range.